Ground State Structural Anomalies in Cuprous Halides: CuCl

Abstract

We have examined structural instabilities and elastic anomalies in CuCl via first-principles calculations and find that its crystal structure is not ideal zinc blende but that correlated displacements of groups of four Cu atoms lead to a more stable configuration. About 20% of the Cu atoms are estimated to be affected. The Cu-Cu distance in the Cu 4 complex shrinks from 3.82 to 2.72 Å, close to that of metallic Cu. The energetics of defect formation are examined also for CuBr and CuI. PACS numbers: 61.50.Ah, 61.66.Fn, 61.72.Ji, 62.20.Dc Cuprous halides are the most ionic crystals with a zinc blende lattice structure. Their ionicities are near the critical threshold above which cubic binary compounds prefer either the NaCl or the CsCl structure [1]. Among the cuprous halides CuCl is the most puzzling and exhibits the greatest number of anomalies. Infrared and Raman experiments show that the phonon spectrum is highly unusual in that two distinct sets of TO and LO phonon modes [2-10] are seen (even at temperatures as low as 2 K) where only one set is expected [11]. The Lyddane-Sachs-Teller relation is not obeyed, unless both sets of phonons are taken into account [2,3]. Another anomaly first pointed out by Martin [12] is that in comparison with other zinc blende compounds the values of the bulk and shear moduli for CuCl deviate strongly from a simple d 24 scaling with bond length. In particular, the bulk modulus of CuCl is only 60% as large as its expected value.It is well known that CuCl is a superionic material at temperatures above about 450 ± C. At lower temperatures, e.g., room temperature, x-ray and neutron diffraction studies show that CuCl is structurally disordered [5,13,14] with the Cu ions having large displacements away from their tetrahedral sites. Several models [3][4][5][6]13,15] have been suggested to explain the phonon anomalies and disorder in CuCl. Vardeny and Brafman [6] proposed the existence of low energy secondary minima along the four ͓111͔ antibonding directions for Cu. The model received strong support from the first-principles calculations of Wei, Zhang, and Zunger who verified the existence of such minima with a formation energy of 0.1 eV [16].The ͑111͒ soft modes can manifest themselves in two different ways. The displaced Cu atoms can occupy the various ͑111͒ minima randomly, giving a statistically disordered distribution; or if the energy barriers are small (as indicated by the calculations), the potential experienced by a Cu ion would be strongly anharmonic and the Cu atoms would sample the four minima dynamically [6,13]. Most experimental data on CuCl have been interpreted in terms of either the statically disordered or the anharmonic model. Schreurs, Mueller, and Schwartz [14] pointed out many years ago that the two models could be ex-perimentally distinguished via neutron scattering experiments. They suggested that for the anharmonic model all diffuse scattering would be inelastic while for the disordered model there would be an elastic component. The results of their experime...

Raman Scattering in CuCl under Pressure

“…According to the first-principles calculations of Park and Chadi [10] the concentration of Cu ions in off-center positions should increase under pressure. This applies to correlated displacements of groups of four Cu ions (Cu 4 defects) as well as other cation displacements.…”

“…In the OCM a sizeable fraction of Cu ions is assumed to occupy offcenter positions [7,8], resulting in a contribution of local vibrational modes to the Raman response. The OCM is supported by results of recent ab-initio calculations [9,10].…”

Pressure-Induced Disappearance of the Raman Anomaly in CuCl

“…Off-center positions are predicted to become energetically more favorable under pressure [3], a fact which should lead to an intensity gain of local vibrational modes. Within the FRM, a major change is expected in the TO-like Raman response because of a detuning of the resonance due to different pressure coefficients of optical and two-phonon acoustic branches.…”

“…Recent ab initio calculations by Wei et al [2] predict metastable off-center minima along [111] and therefore support this model. According to the first-principles calculations of Park and Chadi [3] various off-center defects should even be stable, including Cu 4 groups formed by displacing four neighboring Cu ions towards each other. On the other hand, the effects of isotope substitution were recently found to be consistent with the FRM [5].…”

“…It exhibits several unusual properties, e.g., a large negative thermal expansion at low temperatures, a decrease of the elastic shear constants with increasing pressure, large mean square displacements of Cu atoms even at low temperature, and a high value of the ionic conductivity at high temperatures [1]. Another highly unusual feature, which has attracted recent interest [2][3][4][5], is the anomalous firstorder Raman spectrum [5][6][7][8]. Instead of narrow Raman lines corresponding to the characteristic transverse optical (TO) and longitudinal optical (LO) modes of ZB-type crystals, CuCl shows, in the region of the TO mode frequency, a broad structure consisting of three maxima with TO-like polarization characteristics [6].…”